1. Technical Field
An embodiment of the present invention relates to a semiconductor power device of a multi-drain type and to a corresponding edge-termination structure.
2. Discussion of the Related Art
In the last few years, a wide range of solutions have been developed for improving the efficiency of semiconductor power devices, and in particular for obtaining an increase of the breakdown voltage and a decrease of the output resistance.
For example, U.S. Pat. Nos. 6,228,719, 6,300,171, 6,404,010, and 6,586,798, which are incorporated by reference, describe vertical-conduction semiconductor power devices of the multi-drain (MD) type, wherein, within an epitaxial layer forming part of a drain region having a given type of conductivity, charge-balance columnar structures are provided, having opposite conductivity. These structures have a dopant concentration substantially equal and opposite to the dopant concentration of the epitaxial layer in such a way as to provide a substantial charge balance. Charge balance enables high breakdown voltages to be obtained, and moreover the high concentration that the epitaxial layer can consequently assume enables a low output resistance (and reduced losses in conduction) to be achieved.
In order to manufacture the columnar structures, a sequence of steps of growth of epitaxial layers of a first conductivity, for example of an N type, is envisaged, each step being followed by an implantation of dopant of the second conductivity, in the example of a P type. The implanted regions are stacked and subjected to a subsequent process of diffusion of the dopant atoms so as to give rise to uniform columnar structures.
Next, body regions of the power device are formed in contact with the columnar structures, in the active region, in such a way that the columnar structures constitute an extension of the body regions within the drain region.
The evolution of this technology has shown a progressive increase in the density of the elementary strips forming the devices in order to increase further the charge concentration of the epitaxial layer and obtain devices that, given the same breakdown voltage (substantially linked to the height of the columnar structures), would present a lower output resistance. On the other hand, however, the increase in the density of the elementary strips has led to a reduction of the thermal budget of the devices and a corresponding increase in the number of steps of epitaxial growth, and hence an increase in the manufacturing costs and time and in the defectiveness intrinsically linked to the epitaxial growth.
Alternative techniques have hence been developed in order to obtain the charge-balance structures, which envisage, for example, formation of trenches within the epitaxial layer and subsequent filling of the same trenches with semiconductor material appropriately doped to achieve the charge balance. For example, in patent applications WO 2007/116420 and WO 2007/122646, which are incorporated by reference, techniques are described for obtaining charge-balance structures in electronic power devices, which envisage the formation of trenches and filling thereof without any residual defectiveness via a particular technique of non-selective epitaxial growth performed in the same trenches.
t is also known that the formation of efficient edge-termination structures may be a key point for ensuring proper operation of the power devices. In fact, it is at the edge areas (i.e., the areas surrounding the active area in which the electronic components are provided) that the highest number of breakdown phenomena occurs on account of the thickening of the electric field lines due to the presence of discontinuities, such as sharp edges or the curvature of the doped regions. Edge terminations have the function of reducing the intensity of the electric field locally so as to prevent peaks of intensity at the edges.
FIGS. 1-4 (which are not drawn to scale, as the subsequent figures are also not to scale) show an example of an edge-termination structure of a known type, for a vertical-conduction charge-balance power device. In particular, FIG. 1 shows a schematic and simplified top plan view, whilst FIGS. 2, 3 and 4 show cross sections taken along lines II-II, III-III and IV-IV indicated in FIG. 1, respectively.
The power device, designated by 1, is formed in a die 2 of semiconductor material, for example silicon. The die 2 has, in top plan view, a generically rectangular or square shape; the borders and edges of the die 2 correspond to the so-called “scribe lines” (designated by LT), at which the starting wafer of semiconductor material has been cut. In the die 2 it is possible to define a peripheral portion 2a, adjacent to the scribe lines, and a central portion 2b, in which the power device 1 is physically provided.
The die 2 comprises a substrate 3 having a first type of conductivity, for example of an N++ type, and an epitaxial layer 4, formed on the substrate 3, also having the first type of conductivity, in the example of an N type. Within the epitaxial layer 4 it is possible to distinguish an active area 4a, designed to house elementary electronic components 50 (in the example, MOS transistors) of the power device 1, and an edge area 4b, designed to house an edge-termination structure of the device and adjoining the peripheral portion 2a of the die 2. In particular, the epitaxial layer 4 constitutes a common drain surface region for the plurality of elementary electronic components 50 (the MOS transistors) forming the power device 1.
The edge-termination structure comprises a ring region 5, in particular a region doped with a second type of conductivity, of a P type, with low concentration, for example, lower than 1016 at/cm3, formed in a surface portion of the epitaxial layer 4. The ring region 5 is provided within the edge area 4b, surrounds the active area 4a completely (forming a ring around it), and has an area of superposition with a peripheral portion of the same active area 4a. In particular, the ring region 5 has a rounded-off and curved profile in such a way as to reduce local concentrations of the field lines.
Charge-balance structures 7 (which have in cross section a column conformation, see in particular FIGS. 2-4) traverse the epitaxial layer 4 substantially throughout its thickness, stopping at a certain distance from the substrate 3, both at the active area 4a and at the ring region 5 in the edge area 4b. The charge-balance structures 7 are, for example, obtained through successive steps of epitaxial growth and implantation of dopant atoms in order to obtain stacked doped regions, and through a final step of diffusion of the dopant atoms.
The charge-balance structures 7 follow the layout of the regions in which they are formed, and are constituted by doped regions having the second type of conductivity (P) and a doping level such as to create a substantial charge balance. In particular, in the active area 4a the charge-balance structures 7 are constituted (in plan view, see FIG. 1) by first strips 7a, having a substantially rectilinear extension, parallel to one another and to a first side of the die 2 (and to a first axis x), which repeat periodically and at substantially the same distance in a direction parallel to a second side of the die 2 (and to a second axis y, orthogonal to the first axis x). Instead, in the edge area 4b the charge-balance structures 7 follow the pattern and the profile of the ring region 5, within which they are housed, and are made up of second strips 7b, once again parallel to and set at substantially the same distance from one another, each of which is constituted by: a first rectilinear portion parallel to the first side of the die 2 (and to the first axis x); a second rectilinear portion parallel to the second side of the die 2 (and to the second axis y); and a curved connecting portion between the first rectilinear portion and the second rectilinear portion (in particular having substantially the same radius of curvature as the ring region 5).
In particular, given their columnar extension in the thickness of the epitaxial layer 4, the charge-balance structures 7 constitute vertical walls or diaphragms extending in strips within the same epitaxial layer 4. In addition, in current design rules, the number of the second strips 7b that occupy the ring region 5 is determined by the dimension of the ring and by the pitch (in terms of spacing and size) of the first strips 7a in the active area 4a. 
Body wells 9 are present within the active area 4a, having the second type of conductivity (P) and contacting each first strip 7a of the charge-balance structures 7, at the surface portion of the epitaxial layer 4. In particular, the first strips 7a constitute extensions of the body wells 9 within the drain region in the epitaxial layer 4. Source regions 10, having the first type of conductivity (N), are provided inside each body well 9. In particular, in the area of superposition between the active area 4a and the ring region 5, the outermost body wells 9 join the same ring region 5. In addition, in the edge area 4b, the second strips 7b of the charge-balance structures 7 are joined to one another by the ring region 5.
The power device 1 further comprises, on the surface of the epitaxial layer 4, a first dielectric region (for example, made of silicon oxide) 12, having a greater thickness at the edge area 4b and a smaller thickness in the active area 4a, where it provides the gate-oxide regions of the elementary electronic components 50. A gate region (made of polysilicon or other conductive material) 14 is provided on the first dielectric region 12; on the gate-oxide regions, the gate region 14 provides the gate structures of the elementary electronic components 50.
In addition, a second dielectric region (for example, made of field oxide) 15 covers the first dielectric region 12 and the gate region 14. The second dielectric region 15 is traversed, at the edge area 4b, by a gate metal contact 18, designed to contact the gate region 14. In addition, the second dielectric region 15, the first dielectric region 12, and the gate region 14 are traversed, in the active area 4a, by a source metal contact 16, extending to contact and short-circuit the source regions 10 and the body wells 9. At the periphery of the edge area 4b (adjacent to the peripheral portion 2a of the die 2), the surface of the epitaxial layer 4 is left exposed so as to enable an equipotential-ring (EQR) metal contact 19 to contact a doped region 20, in particular a doped region having the first type of conductivity (N), provided in the surface portion of the epitaxial layer 4. The doped region 20 is set at a distance from the ring region 5, and has the same ring layout as the latter, surrounding it completely. The contact region 20 has the function of bringing to the surface the drain potential so as to limit horizontally the electric field lines in reverse biasing.
In analyzing the cross sections of FIGS. 2-4, it is to be noted in particular that the cross section of FIG. 2 is taken in a direction transverse to the direction of extension of the charge-balance structures 7, and that the cross sections of FIGS. 2 and 3 are both taken along the direction of extension of the charge-balance structures 7, but on the outside and on the inside, respectively, of a first strip 7a. 
It has been shown that power devices of the type described, although having considerable advantages as compared to traditional solutions, may be subject to phenomena of early breakdown that can jeopardize their performance or, in the worst case, prevent their subsequent use (i.e., destroy them).